1. Field of the Invention
A compressor assembly and method of fabricating the same wherein a coating interacts between pistons and the drive shoes that reciprocate the piston.
2. Description of the Prior Art
In a swash plate type compressor used in air conditioning systems, the rotation of the swash plate is converted to the reciprocating movement of the pistons through respective shoes. The shoe is a semi-spherical part that has a flat surface in contact with the swash plate and spherical surface in contact with a ball pocket in the piston. In response to rotation of the angled swash plate, the shoe transfers the load to the piston, which forces the piston to move reciprocally in a cylinder as the spherical surface of the shoe slides against the ball pocket surface in the associated piston. Typically, the shoe is made from hardened steel and the piston is made from an aluminum alloy. Under high-load and high-speed compressor operating conditions, the shoe transfers significant sliding wear load to the surface of the ball pocket. This high sliding wear load can deform and/or tear the relatively soft aluminum alloy surface of the ball pocket. Thus, galling or seizure at the shoe to the ball pocket interface can occur. This galling tendency can be accelerated and made more severe under a lack of lubrication condition that can result in the failure of the compressor. Therefore, a protective coating at the surface of the ball pocket is necessary to prevent galling or seizure at the shoe to ball pocket interface. In general, an ideal ball pocket coating should provide the following characteristics:                a) Conformability:—To compensate for irregularities in the surface of the ball pocket and provide a uniform contact area. This characteristic will act to reduce the wear load stress concentration.        b) Lubricity:—To provide a low coefficient of friction at the surface of the ball pocket in order to reduce wear and frictional heat generation.        c) Excellent adhesion to the substrate for extended coating life        d) Durability against premature loss of function.        
Traditionally, coating of the surface of the piston pocket with tin is a widely used practice for providing the four characteristics outlined above. Normally applied by an immersion process, or a chemical conversion process, the tin coating can provide for a good surface break-in and a certain degree of self-lubrication to thereby reduce the galling tendency at the surface of the piston pocket. However, the tin coating has certain limitations. First of all, the tin coating does not provide adequate protection against the galling tendencies under all circumstances. For example, under certain low lubrication condition at the shoe to pocket interface, ball pocket galling/seizure can still occur with the tin coating present. In some situations, the tin coated/plated ball pocket remains the primary failure mode of the compressor during low/no oil operation. A second limitation to the tin coating process is related to the environmental issues associated with process wastewater treatment. The separation of heavy metal from the wastewater is difficult and costly. In some manufacturing facilities, local environmental regulations prohibit the use of the tin coating process thereby requiring a remote site to apply the tin coating. This drives the need for additional inventory and work-in-process to compensate for the logistics required to use an outside or remote source, resulting in limitations in piston manufacturing process efficiency and negatively impacting total cost. In addition to this process limitation, the increase of environmental regulations globally will inevitably lead to an increased cost for wastewater treatment, and result in a more expensive tin coating process in the future.
In order to improve performance, increase productivity, reduce environmental impact and lower the piston production costs, alternative coating technology have been sought to replace the current tin coating on the ball pocket of the piston.
A polymer based solid lubricant coating is an attractive solution due to its much lower coefficient of friction as compared to the tin coating; however, the ball pocket coating requires a very thin coating layer (2-4 um), and it is difficult to apply the polymer-based coating in such a thin layer with the desired coating properties. In such a thin layer, the polymer-based coating typically does not adhere very well to the base substrate and will be worn very quickly.